2/06/2004 ALADIN Workshop, Innsbruck 1

The general problem of moist processes in ALADIN-2

J.-F. Geleyn

ALADIN-2/CHMI (Prague) & Météo-France (Toulouse)

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A few obvious (?) statements

At least for our latitudes, the current ARPEGE-like moist physics (ACCVIMP(D), ACPLUIE, LCVPP, ACNEBN) doesn’t work so bad at the ~10 km scale (since DIFCORA + COCONUT).There is a huge paradox in the choice of a strategy oriented around the existence of the ‘grey-zone’, together with the affirmation that the same physics can be the best solution on each side!There is a true consensus around the choices for AROME.It would be stupid to redevelop the parts that really differ between AROME and ALADIN-1 (microphysics, turbulence).For other aspects relevant to ALARO, science should progresses from ‘doubt’ and ‘confrontation of ideas’. Else we are doomed to revive the ‘dynamical kernel’ syndrome.

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The constraints

Owing to what was just said, the scientific degrees of freedom are restricted to ALARO, but they are conditioned to a large extent by choices in AROME and even in ARPEGE => unstable situation (cf. WG discussion of 1/6).For ALARO, efficiency (i.e. mostly the length of the physics time-steps) ought to be paramount => it is not sufficient to think of it in absolute terms (e.g. ‘60 s’), one must count in relative terms => contradiction with AROME?Keeping the possibility to solve the ‘grey-zone’ problem is a key part of the challenge, but it should not lead to additional contradictions => convergence of constraints at the most delicate point!

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Basic proposals

The organisation of the phys-dyn time-step must be thought in terms of SI-SL spectral schemes, and be studied alike.For the way to combine effects of various parameterisation schemes one should work from the general to the special:

The set of general equations (the full-barycentric choice is a good offspring of the need to find compromises);Its consequences on getting and combining tendencies (interface);The latter’s consequences on individual routines for full consistency.

For (may-be) doing better for the grey zone problem, 3 ideas:Prognostic convection;One single approach to phase changes (adapt. AROME micro-phys.);Mixed input for downdraft (resolved + updraft).

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All this starts to resemble to a complex road-map, with constraints and

opportunities.

Dry turbulence (Meso-NH)

Cloud input for radiation (t. b. d.)

Micro-physics (Meso-NH)

Convection Prog. convection

?

?

Non precipitating convection

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Priorities

To find a consensus on the equations and on their AROME-ALARO-(ARPEGE?) basic declination.To start looking at the problem of the micro-physics time-step-length (algorithmically, not scientifically).To prepare testing of deep-convective ideas in a very wide sense (if one wants compatibility between ALARO-10 and ALARO-5 in methods and between AROME and ALARO-5 for micro-physics, cross-fertilisation will be necessary).To consider ab-initio the non-precipitating convection problem in the same spirit, but across even more scales! Hopefully the transition with dry-turbulence will be more crucial than the one with deep convection. Otherwise => big hurdle!!

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Conclusions

Obeying the list of constraints may look like an impossible challenge.This may fortunately not be so, if one accepts a few basic principle for development and if the initial priorities are the right ones.But this requires a wide consensus that obviously does not (yet?) exist, on either side (M-F/Partners).This point may a-priori look academic, but in fact it is central to the question of the adequation of ALARO to ALADIN Partners’ real needs.